The present invention relates generally to photolithography masks, and more particularly to attenuated phase shift masks. Still more particularly, the present invention relates to methods for repairing defects in an attenuated phase shift mask.
In a manufacturing process using a lithographic projection apparatus, a mask pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as, priming, resist coating, and soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake, and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc.
The basic lithography system consists of a light source, a stencil or photomask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. Conventional photomasks consists of chromium (Cr) patterns on a quartz plate, allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask onto the photoresist coated wafer, exposing the resist wherever hole patterns are placed on the mask. Exposing the resist to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers which, in common applications, allow a developer to dissolve and remove the resist in the exposed areas. Such resist materials are known as positive resists. (Negative resist systems allow only unexposed resist to be developed away.) The photomasks, when illuminated, can be pictured as an array of individual, infinitely small light sources which can be either turned on (points in clear areas) or turned off (points covered by chrome). If the amplitude of the electric vector which describes the light radiated by these individual light sources is mapped across a cross section of the mask, a step function will be plotted reflecting the two possible states in which each point on the mask can be found (light on, light off).
The quality with which small images can be replicated in lithography depends largely on the available process window. That is, the amount of allowable dose and focus variation that still results in correct image size. Phase shifted mask (PSM) lithography improves the lithographic process window or allows operation at a lower k value by introducing an additional parameter on the mask, i.e., an electric vector. The electric vector, like any vector quantity, has a magnitude and direction, so, in addition to turning the electric field amplitude on and off, it can be turned on with a phase of about 0 degree or turned on with a phase of about 180 degree. This phase variation is achieved in PSMs by modifying the length that a light beam travels through the mask material. By recessing the mask to an appropriate depth, light traversing the thinner portion of the mask and light traversing the thicker portion of the masks will be 180° out of phase, that is, their electric field vector will be of equal magnitude but point in exactly the opposite direction so that any interaction between these light beams result in perfect cancellation.
In recent years, the phase shift mask (PSM) has been gradually accepted by the industry as a viable alternative for sub-exposure-wavelength manufacturing. Two fundamental forms of PSM have been used the most, namely alternating PSM (altPSM) and attenuated PSM (attPSM). In the attenuated phase shift mask, the surface is mainly divided into two regions, which are the wholly transparent region in 0 degree phase and the attenuated transparent region in 180 degree phase. The wholly transparent region is mainly constructed of quartz, and the attenuated transparent region has an extra molybdenum silicide (MoSi), or similar material, layer. The transparency of the wholly transparent region is close to 100%. The transparency of the attenuated transparent region is much less than that of the wholly transparent region, and is typically less than 10%, possibly about 4 to 6%. The light that arrives at the photoresist on a wafer through the wholly transparent region of the photomask is 180 degrees out of phase with light that arrives at the photoresist on the wafer through the attenuated transparent region of the photomask. Where the two pattern regions appear adjacently, the phase difference produces destructive interference. The contrast between the wholly transparent region and the attenuated transparent region can be more pronounced, so that the resolution of the exposure process can be improved.
Several types of defects are possible in the production process for attPSM. There may have been defects in the original Cr coating, defects in the original resist coating, particles on the resist coating, Cr peeling, or electron-beam (or laser beam) writing errors. All these possible problems may show up in the delineated Cr pattern and therefore, in the delineated mask pattern. Accordingly, there remains a need for a repair process that can restore the designed pattern.